1. Field of the Invention
The present invention generally relates to solid-state imaging devices, image sensors, image processing apparatuses, and imaging methods, and particularly relates to a solid-state imaging device that performs predetermined signal processing after acquiring image signals by use of an image sensor, as well as an image sensor, an image processing apparatus, and an imaging method for use in such solid-state imaging device.
2. Description of the Related Art
In solid-state imaging devices equipped with an image sensor such as a CCD or CMOS, signals output from the image sensor are subjected to predetermined signal processing for generation of image signals suitable for screen display, and these image signals are supplied to a next stage. Such signal processing performed in solid-state imaging devices includes defect pixel correction for correcting defects by processing the data of defect pixels contained in image signals, color interpolation for obtaining color data for each pixel based on color information obtained from a RGB Bayer pattern, shading correction for correcting the distortion of a lens based on the color data, auto white balance, gamma correction, edge processing, etc.
FIG. 1 is a block diagram showing an example of the construction of a related-art solid-state imaging device. The solid-state imaging device of FIG. 1 includes an image sensor 10 and an image-processing unit 11. The image sensor 10 supplies an acquired image signal as digital data to the image-processing unit 11, together with a horizontal synchronizing signal, a vertical synchronizing signal, and a clock signal. The image-processing unit 11 performs various signal processing as described above to generate digital image data for output on a screen, and supplies the data to a next stage, together with a horizontal synchronizing signal, a vertical synchronizing signal, and a clock signal.
The image sensor 10 includes a pixel array 21 of solid-state image sensing elements such as a CCD or CMOS, a noise cancellation circuit (CDS) 22, an amplifier 23, an analog-to-digital converter (ADC) 24, and a timing generator 25. The pixel array 21 is comprised of a plurality of photodiodes arranged in a matrix form to serve as a light sensing unit. These photodiodes correspond to individual pixels for the imaging purpose. Incident light is subjected to optoelectronic conversion on a pixel-by-pixel basis, and electric charge obtained through optoelectronic conversion is accumulated in a charge built-up portion for signal read. The noise cancellation circuit 22 reads an image signal from the pixel array 21 while reducing noise by correlated double sampling, for example. The obtained image signal is amplified by the differential amplifier 23, and is converted from an analog signal into a digital signal by the analog-to-digital converter 24.
The timing generator 25 generates the horizontal synchronizing signal, the vertical synchronizing signal, and the clock signal in synchronization with the digital image signal, and supplies these signals and the digital image data to the image-processing unit 11. The digital image data are divided into chunks such that image data corresponding to one individual horizontal line constitute one data chunk, and image data corresponding to individual horizontal lines are successively output chunk by chunk. Between the individual horizontal lines, a blanking period is provided where no image data exists, and serves as a margin for use for aperture adjustment or the like. The horizontal synchronizing signal indicates a valid data period for the image data corresponding to individual horizontal lines. The horizontal synchronizing signal becomes HIGH during a period where the data of each horizontal line exists, and becomes LOW in other periods.
The image-processing unit 11 applies various signal processing as described above to the supplied image data during a period in which the horizontal synchronizing signal becomes HIGH, thereby generating digital image data for output on a screen. The image-processing unit 11 further outputs the vertical synchronizing signal, the horizontal synchronizing signal, and the clock signal in synchronization with the output image data.
FIG. 2 is a timing chart showing timing relationships between various signals input into the image-processing unit 11 and various signals output from the image-processing unit 11. In FIG. 2, (a) designates a vertical synchronizing signal on the input side, (b) designating a horizontal synchronizing signal on the input side, and (c) designating image data on the input side. Further, (d) designates a vertical synchronizing signal on the output side, (e) designating a horizontal synchronizing signal on the output side, and (f) designating image data on the output side. As shown in FIG. 2, image data for n lines corresponding to a first line to an n-th line are supplied during a period in which the vertical synchronizing signal is HIGH. Image data corresponding to one line is equivalent to m pixels, so that the entirety of the image data corresponds to an m×n pixel array. Further, the horizontal synchronizing signal is supplied in synchronization with the image data, becoming HIGH during the valid period of the image data corresponding to each line. As shown as a processing latency in FIG. 2, there is a delay between the signals on the input side and the signals on the output side of the image-processing unit 11. This is because the image-processing unit 11 performs signal processing on the image data separately for each block, so that the image data is not processed until all the data of a given block is input into the image-processing unit 11.                [Patent Document 1] Japanese Patent Application Publication No. 2000-188721.        
In order to reduce power consumption, the image-processing unit 11 does not perform signal processing during the blanking periods (periods between adjacent horizontal lines). When signal processing by the image-processing unit 11 starts after the processing latency as shown in FIG. 2, therefore, power-supply noise or the like caused by the running of the digital circuit affects the operation of an analog circuit of the image sensor 10.
FIG. 3 is an illustrative drawing for explaining an effect of the operation of the image-processing unit 11 on an analog circuit. In FIG. 3, (a) shows the horizontal synchronizing signal output from the image sensor 10 (i.e., the horizontal synchronizing signal input into the image-processing unit 11), (b) showing image data output from the image sensor 10 (i.e., the image data input into the image-processing unit 11), (c) showing a noise level, (d) showing the horizontal synchronizing signal on the output side of the image-processing unit 11, and (e) showing the image data on the output side of the image-processing unit 11. Here, the noise level refers to noises occurring in the power supply that is commonly used by the image sensor 10 and the image-processing unit 11.
As shown in FIG. 3, an analog circuit of the image sensor 10 operates to output the image data shown in (b). While the image data is being output, the image-processing unit 11 starts its operation at the timing (corresponding to a rise of the horizontal synchronizing signal illustrated in (d)) that is delayed by the processing latency from the timing of a rise of the horizontal synchronizing signal shown in (a). As a result, the noise level shown in (c) changes into a larger noise level as the image-processing unit 11 starts its operation. This change in the noise level affects the operation of the analog circuit of the image sensor 10, so that the image data of first X pixels of the horizontal line and the image data of subsequent pixels have different levels, i.e., have a code displacement as shown in (b). Here, the number X is the number of pixels corresponds to the processing latency.
In this manner, the digital image data should be maintained generally at a fixed level in the output of the image sensor 10, but there is a code difference between before and after the start of operation due to noises caused by the operation of image-processing unit 11. Consequently, the image data output from the image-processing unit 11 after signal processing ends up having a stripe that has different brightness extending in a vertical direction of the screen with its width equal to X pixels.
Accordingly, there is a need for a solid-state imaging device in which image data is not affected by a change in the noise level caused by the start of operation of an image processing unit, and is also a need for an image sensor, an image processing apparatus, and an imaging method for use in such solid-state imaging device.